Relay stations (RN: Relay Node) studied under the 3rd Generation Partnership Project (3GPP) are constantly connected to a given base station (eNB: evolved-NodeB) (see, for example, 3GPP TR36.912 9.0.0 Chapter 9 “Relaying” and 3GPP TR36.806 “Relay Architectures for E-UTRA (LTE-Advanced)”). Therefore, load may concentrate at the given base station, exceeding the processing capability of the base station and causing a congested state of the base station.
At present, congestion control by the base station to deal with such a problem includes reducing through-put of data traffic by a Radio Resource Management (RRM) function, handing over a link to another base station, changing an audio codec, and cutting off a call of low priority (see, for example, 3GPP TS36.300 9.0.0 Chapter 16 “Radio Resource Management Aspects”).
Conventional congestion control, however, may lead to lower service quality or failure in providing service. One conceivable method for preventing such a problem is to allow a relay station to change base stations. It is difficult for the relay station, however, to properly select a base station to which communication is to be switched when the base station currently communicating with the relay station enters a congested state. For example, when, as a communication counterpart, a base station is changed over to another base station, the other base station may be found to be in a congested state or the change over to the base station may cause the base station to enter a congested state. In such a case, base station changeover is carried out again, which causes a delay and leads to the deterioration of communication quality.